FB just recently changed their format and it is a bit of a challenge. To change a post that you have made there are 3 dots ... along side the right hand side of the message you want to edit or delete. The dots are very faint. That is what I would try to start with. When it comes to staying in a specific group you want to enter it with the group titles on the left hand side of your screen where you might normally enter it our by using the group icon in the middle of the screen. This one can be tricky as it will let you into all the group conversations if you do not enter the appropriate group.
Now I hope this makes a bit of sense. I am not real good at explaining things I just hope this helps a bit.